Preconditioning limits mitochondrial Ca²⁺ during ischemia in rat hearts: Role of K_ATP channels

Prolonged myocardial ischemia results in an increase in intracellular calcium concentration ([Ca²⁺]_i), which is thought to play a critical role in ischemia-reperfusion injury. Ischemic preconditioning (PC) improves myocardial function during ischemia-reperfusion, a process that may involve opening mitochondrial ATP-sensitive potassium (K_ATP) channels. Because pharmacological limitation of mitochondrial calcium concentration ([Ca²⁺]_m) overload during ischemia-reperfusion has been shown to improve myocardial function, we hypothesized that PC would reduce [Ca²⁺]_m during ischemia-reperfusion and that this effect was mediated by opening mitochondrial K_ATP channels. Isolated rat hearts were subjected to 25 min of global ischemia and 30 min of reperfusion with or without PC in the presence of mitochondrial K_ATP channel opening (diazoxide, 100 μM) and blockade [5-hydroxydecanoic acid (5-HD), 100 μM]. Contracture during ischemia (end-diastolic pressure) and functional recovery on reperfusion (developed pressure) were assessed. Total [Ca²⁺]_i and [Ca²⁺]_m were measured using indo 1 fluorescence. Both PC and diazoxide limited the increase in end-diastolic pressure and resulted in greater functional recovery after 30 min of reperfusion, functional effects that were partially or completely abolished by 5-HD. PC and diazoxide also significantly limited the increase in [Ca²⁺]_m during ischemia-reperfusion. In addition, PC lowered [Ca²⁺]_i during reperfusion, whereas diazoxide paradoxically resulted in increased [Ca²⁺]_i during reperfusion. There was an inverse linear relationship between [Ca²⁺]_m and developed pressure during reperfusion. PC limits the ischemia-induced increase in mitochondrial, but not total, [Ca²⁺]_i, an effect mediated by opening mitochondrial K_ATP channels. These data suggest that the lowering of mitochondrial calcium overload is a mechanism of cardioprotection in PC.